Three years ago, Britain had a new Prime Minister and a promise to take the United Kingdom out of the European Union by delivering on the people's vote for "Brexit".

"I had the feeling that our British counterparts thought it would be a very easy thing to do." Martin Selmayr, outgoing Secretary Gen of the European Commission

What had been presented as a simple proposition - for the UK to leave - has turned into a political nightmare, dividing former allies, British political parties and the people of an increasingly dis-United Kingdom.

"Brexit is a negative negotiation, it is a lose lose game for everybody." Michel Barnier, European Chief Negotiator

Theresa May failed to deliver, and it cost her the top job.

"The real issue is debating with ourselves what kind of Brexit Britain wants. And we should have done that before we triggered the process." Philip Hammond, Chancellor 2016-19

Now Britain's new PM is making bold promises to "leave". But Boris Johnson will face the very same challenges.

"Let's hope he gets us out of the European Union...Frankly if we don't it's the end of the Conservative Party, it's the end of his premiership." Richard Tice, Chair, Brexit Party

In Britain, those at the heart of the Brexit talks are trying to make sense of how they got to this point and how they will resolve the crisis.

"Ironically some of the most ardent champions of Brexit were guilty of the greatest underestimation of how difficult Brexit would be." David Liddington, UK Minister, 2016-19

In this revealing account, key negotiators and politicians on both sides of the Channel tell the inside story of the Brexit debacle.

Senior players recount missteps and excruciating moments where they were blindsided.

"I was trying to keep my face dead straight, conscious that there were cameras on me. It was a surprise to see that the Prime Minister had chosen to define Brexit in the hardest possible terms." Philip Hammond, Chancellor 2016-19

Also laid bare is the infighting and betrayals within the ruling British Conservative government as the tortuous negotiations continued.

"There was no way I could walk that deal over the line I couldn't do it as a matter of personal conviction and I couldn't do it for the good of my country." Dominic Raab, Former Brexit Minister, current Foreign Secretary

This film from the BBC's Panorama program lays out Britain's troubled path to Brexit. The following week, the ABC's Chief Correspondent Phil Williams will report on what happens next now that Boris Johnson is the man in charge.

"Just a few days ago the Brexit circus that has paralyzed this nation for over three years took another amazing turn. The controversial Boris Johnson is now Prime Minister and it's up to him to somehow resolve the most serious political crisis to grip

the country in decades. If he gets it wrong some fear it could lead to the break-up of the United Kingdom." Phil Williams, ABC Chief Correspondent

Britain's Brexit Crisis, from the BBC's Panorama program, goes to air on Monday 5th August at 8.30pm. It is replayed on Tuesday 6th August at 1.00pm and Wednesday 7th at 11.20pm. It can also be seen on ABC NEWS channel on Saturday at 8.10pm AEST, ABC iview and at abc.net.au/4corners.